This RFA is motivated by the absence of a consistent method for the non-invasive diagnosis of deep vein thrombosis (DVT), a common and potentially serious complication of both medical and surgical patients. This study will be conducted in three parts. In the first, scheduled for years 1 through 3, a cohort of 325 consecutively studied, acute symptomatic patients with suspected deep vein thrombosis will undergo platelet scintigraphy(PS), Ultrasound(U) and impedance plethysmography(IPG) within the time windows of 0-2 hours and 2 to 4 hours after presentation. Ultrasound will include conventional and color flow doppler plus 2 Dimensional imaging. These techniques will be compared to the gold standard of contrast venography(CV). Time windows will be randomly assigned for each technique. CV will be performed 4 to 5 hours after presentation. Particular attention will be paid to pelvic and inferior vena caval thrombi. Femoral vein injections of contrast material will be made if necessary. CV, PS and U will be interpreted by two independent observers. The inter and intra observer variability will be determined. Each technique will then be compared independently to CV and cross comparison between techniques will be made. Analysis will be for the whole limb and for segments below the knee, below and above the inguinal ligament. The effect of anticoagulant therapy will be evaluated. In patients with positive PS, daily images for 7 days of legs and lungs will be obtained to monitor progression, resolution of DVT and/or the development of pulmonary emboli. Calf thrombi, positive by CV, will be monitored for extension by IPG, U for 10 days. In the second part (years 1 and 2) magnetic resonance imaging (MRI) will be evaluated in a dog model of DVT (25/year). These studies will determine the optimum pulse sequence, field strength and compare tomographic imaging with projection angiography. In addition correlations with thrombus age, changes due to heparin therapy will be made. Pelvis thrombi will specifically be studied. A major effort will be made to distinguish slowly flowing blood from thrombosis. The third part of the study, to be conducted in year 3 will evaluate 10 compliant patients with CV proven and untreated venous thrombi by MRI using the technical data derived from part 2. This study will represent the first direct and systematic comparison of P.S., IPG and U against CV. It will also be the first study to monitor progression of CV proven and untreated calf vein thrombosis by IPG and D. It will also systematically evaluate the potential of MRI for thrombus detection.